Design Of Heavy Frequency Z-pinch Driver Based On LTD Technology

Linear-transformer-driver(LTD)has been warmly studied world widely in the past two decades,aiming to build Z-pinch accelerators that can be operated repetitively.Fast high current pulses can be generated directly by LTD,thus the efficiency is significantly increased compared to the traditional technology,which is based on Marx generator,intermediate storage and pulse-forming-line.Meanwhile,since the power loaded in each component is relatively low,a LTD accelerator can work repetitively.Therefore,LTD is reckoned as the chief technology for building future high-power Z-pinch accelerators,and is counted as a key subject in researching of Z-pinch-driven inertial fusion energy.This dissertation was focused on design of a repetitive LTD accelerator coupled to Z-pinch loads.A LTD stage,together with relevant devices and apparatus,were experimentally investigated.A magnetic hydro-dynamics(MHD)code was built,coupled to circuit model of the generator,and the characteristic of the coupling from generator to Z-pinch load was studied.Design of a 5-MA vacuum-line-based LTD accelerator was performed,and simulations of the accelerator were conducted via a full circuit modeling using TLCODE for transmission in MITLs and 0-D thin shell model for Z-pinch load.In support of the LTD stage development effort,a 6-gap gas switch was designed.Tests results show that the deviation of the coronal currents in each gap is about 5%,and is related to the voltage acted across the gap as a 2.5 power function,thus the deviation of voltages between each gap is about 2%.The breakdown voltage of the switch agrees well to the expected value from experiential formula,and the delay-time between trigger pulse and the output current is 55.0±1.3ns(lσ).An extensible trigger pulse generator was designed,based on pulse forming technology using high voltage cables.The pulse outputted to a 2-kΩ resistor is 150kV in voltage amplitude,15ns(0.1-0.9)in rise-time and 100ns in FWHM.Optimizations of capacitors were carried out,and the accelerated lifetime tests were conducted.The lifetime of a capacitor that withstands 120kV is 248%higher than one withstands 10OkV.An 800-kA LTD cavity was built and tested for more than 6000 shots intermittently at a rep-rate of 0.1 Hz.A novel trigger way,in which only one input trigger pulse is needed was developed and successfully verified in these shots.Using this trigger method,the number of trigger pulses needed by LTD stages can decrease by 75%.The cavity used triggering and charging inductors,which shows great promise in life-time compared to liquid and solid resistors,which would develop air bubbles or be shattered occasionally.A 1-D MHD code for Z-pinch implosion was built,together with a simplified circuit modeling for the generator.Using this code,the influence to electro-magnetic pulse coupling and energy transmission by the inductance in the vacuum area between transmission line and the Z-pinch load,as well as parameters of loads and transmission line,were investigated.By increasing the vacuum inductance in a limited range,a better match for peak power from the transmission line to the load can be achieved.Moreover,this results in an increase of the energy stored in the vacuum inductance,which will be released and participate in the Z-pinch implosion after peak current time.The contribution of the vacuum inductance energy to the increase of load energy can be more than 40%.Furthermore,using bigger but lighter loads,the output current,as well as the implosive velocity and kinetic energy,can be increased.A high implosive velocity results in a short thermolization time of the staged plasma,generate a high power X-ray pulse.For a small-sized LTD accelerator,the short transmission line between pulse source and Z-pinch load can be treated as an inductor,leading to an increase of the circuit inductance and rise time of the current pulse,as well as a decrease of the peak current.However,since the reflected energy from the load can be return and participate in the Z-pinch implosion,the energy efficiency of the accelerator can be improved by increase the implosion time using a weighty load.Based on architecture of the 800-kA cavity,we performed a design for a Z-pinch accelerator composed of 6 modules,each consisting of 8 stacked annular LTD cavities in a voltage-adding configuration.All the transmission lines utilized in this design work in vacuum,thus the high voltage insulating stack is eliminated.Besides,the accelerator is 3.2m tall and 12m in outer diameter including supports,which is appreciably compacter than accelerators with similar output but using traditional technology.An equivalent circuit model for electromagnetic pulse transmission in MITLs as well as in voltage adders was developed.The 0-D thin shell model was employed to characterize the pinch load.Simulation results shows that power flow in the vacuum lines with minimum AK gap of 1cm behaves well,and the output current can be as high as~5MA with a rise time of～100ns.Especially,an implosion velocity of~30cm/μs can be generated,leading to an energy efficiency of 11-14%.According to scaling experiment results obtained at Angara-5-1 and Saturn,the efficiency from stored electric energy to radiation can be as high as 20-30%,which nearly doubles the figure ever obtained currently in fast Z-pinch experiments.It therefore provides a high efficient facility for research on fast Z-pinch and technologies for rep-rately operated accelerators.In conclusion,the study of this dissertation improved and examined the performance of the LTD technology for repetitive operations.The characteristic of coupling from the generator to Z-pinch load was investigated,the results of which can be a beneficial reference for design of future high-power Z-pinch accelerators.The proposed design of the vacuum-line-based LTD accelerator eliminates traditional flashover problem of the insulation stack,thus can be operated repetitively,and can provide a significant facility for investigations on Z-pinch driven inertial fusion energy.